1. Field of the Invention
The present invention relates to a drill including a cutting edge a point angle of which varies depending on a position, and a drilling method for a workpiece.
2. Description of the Related Art
Examples of the drill including the cutting edge a point angle of which varies depending on a position may be double angle drills disclosed in Japanese Unexamined Patent Application Publication No. 63-306812 (hereinafter, referred to as document '812) and in Japanese Unexamined Utility Model Application Publication No. 06-75612 (hereinafter, referred to as document '612).
Such a double angle drill includes a primary cutting edge and a secondary cutting edge. The tip portion of the primary cutting edge has a tip shape of a metal drill. The secondary cutting edge is flat, is continuous to the primary cutting edge, and has a smaller point angle than that of the primary cutting edge.
Documents '812 and '612 teach that the double angle drill is suitable for drilling a hole in a fiber reinforced plastic composite material and a metal material at a time.
With the double angle drill, the primary cutting edge drills a primary hole with a relatively small diameter, and then the secondary cutting edge cuts the outer periphery of the primary hole to drill a secondary hole with a target diameter. During drilling, transient delamination may appear at the periphery of the primary hole of the composite material. The delamination is removed by cutting the primary hole by the secondary cutting edge.
Unfortunately, as disclosed in document '612, the double angle drill typically has low wear resistance. Document '612 discloses a configuration to increase the wear resistance of the cutting edge by covering the cutting edge with a diamond film with a certain film thickness.
However, the double angle drill has corners at the boundary between the primary and secondary cutting edges and at the outermost periphery of the secondary cutting edge. A stress is likely concentrated at the corners, and hence chipping likely appears at the corners. Thus, such a shape of the drill decreases the wear resistance of the drill.
In addition, to drill a hole in a component member, as a workpiece, in which a metal material and a fiber reinforced plastic composite material are combined, the double angle drill still has problems as follows.
In a situation in which the component member is drilled by the double angle drill from the metal material side, when the fiber reinforced plastic composite material is drilled after the metal material is drilled, the primary cutting edge penetrates through the fiber reinforced plastic composite material while insufficiently cutting but pressing fibers of the fiber reinforced plastic composite material. Then, the secondary cutting edge cuts the fibers. At this time, delamination and fiber fraying are expanded in the component member at the pressed portion. The delamination and fiber fraying expanded in the component member may not be cut even by the secondary cutting edge. Thus, the delamination and fiber fraying may not be removed and may still remain.
In another situation in which the component member is drilled by the double angle drill from the fiber reinforced plastic composite material side, when the metal member is drilled after the fiber reinforced plastic composite material is drilled, the fiber reinforced plastic composite material is cut while being raked by the secondary cutting edge having a rake angle. Hence, burr and fiber fraying may appear at the entrance of the drilled hole.